A common problem with conventional bending apparatus such as press brakes is that when V-bending a sheet-like workpiece by such bending apparatus, the behavior of workpiece plastic deformation varies depending upon the material properties. It is well known in the art that even when the same material is used, the angle of a bend produced in the workpiece is likely to significantly vary from one lot to the next due to the variation in material properties. Because of this, it is extremely difficult to accurately control the amount of penetration of the movable die (punch). Practically, the selection of the punch penetration amount is an artful choice based on trial and error coupled with the experience of the press brake operator.
In order to cope with the forgoing problem, various types of press brakes have been proposed and some of them are now put into practical use, in which a workpiece bend angle is detected during bending and based on the workpiece bend angle thus detected, the amount of punch penetration is controlled. One such example of a press brake is shown in Japanese Patent Publication Kokai Gazette No. 7-265957 (1995). The press brake shown in the patent application No. 7-265957 is based on the fact that the variation in springback angle between material lots is small. Data relating to the springback angle are stratified by workpiece material and by plate thickness for storage. Arithmetic operations are performed on the basis of the stored data to find a final punch penetration amount. An advantage of this prior art control technique is that an estimation of a springback angle can accurately be formed by a single angle detection process without having to perform a complicated control process on the movable die.
Practically, the springback angle varies to some extent because of variations in the material properties between lots (see FIG. 7). FIG. 7 shows springback angles at different bend angles of 6 kinds of cold rolled steel plates. As can be seen from FIG. 7, the springback angle varies within the range of about .+-.0.3 degree because of the lot-to-lot material property value variation. This results in the requirement that when bending a workpiece at an accuracy of .+-.0.3 degree or less, a springback angle must be estimated or detected with satisfactory accuracy.
A method for detecting a springback angle has been known in the art. In this detection method, the upper and lower dies are temporarily and relatively moved away from each other for providing a clearance therebetween during bending, to detect a springback angle from the difference between a pre-separation bend angle and a post-separation bend angle. However, this springback detection method is problematic in that when the upper and lower dies are moved towards each other after the separation, the point of contact of each die with the workpiece changes. This results in poor processing accuracy.
The present invention is directed to overcoming the foregoing problem associated with the prior art technique. Accordingly, a major object of the present invention is to provide an improved bending method and its associated bending apparatus. The present invention makes it possible to compensate for springback angle variation caused by variations in material properties from one lot to the next for the realization of very high-accuracy bending.